1. Field of the Invention
The present invention relates to heat transfer devices, and more particularly heat transfer devices for use with electronic instrumentation and computation systems.
2. Art Background
Heat producing devices including electronic circuit elements commonly require heat dissipation facility to draw away heat sourced by the heat producing components during operation of the system. Frequently, heatsinks having an array of fins or other surface area extending geometry are firmly connected to the heat producing bodies, the thermal mass of the heatsink drawing away heat from the device and the fins transferring the heat to ambient or convective air. More recently, heat pipe technology has become more prevalent in application to commercial or industrial heat transfer applications, particularly where efficient heat transfer must be accommodated in a compact or limited physical space, or where liquid cooling techniques are impractical. Heat pipes, although known for at least twenty years and having their roots in work done in the 1940's, are just recently finding application to general commercial and industrial environments. A heat pipe is essentially a hollow thin-walled thermally conductive material, for example copper, which is evacuated and a small amount of an inert working fluid introduced into the thin-walled element, whereafter the element is hermetically sealed. Once surface of the heat pipe element draws heat away from a heat producing body in contact with the heat pipe element, and transfers it to the sealed working fluid. The working fluid is caused to locally boil, the resultant vapor moving rapidly away from the heated region through passages in the heat pipe to cooler regions where the vaporized working fluid is condensed. Capillary action in a thermal wick applied to interior surfaces of the heat pipe returns the condensed working fluid to the heated region. A fundamental feature of heat pipes is that the entire external surface of a heat pipe contacting a heat producing body may be maintained to within 1.degree. C. for reasonable power densities. The isothermal characteristic of heat pipes is of particular interest to solid state instrumentation or computation electronics, where device junction temperatures must be maintained with in relatively narrow temperature ranges, and wherein temperature differences between chips on one circuit module must not vary significantly.
Although heat pipes have been previously used to cool computer and electronic systems components, the applicability of heat pipes to particular operating environments has been limited due to physical constraints of the heat pipe itself. For example, because a heat pipe element is typically fabricated of a thin-walled metallic material, there exists little if any structural strength in the heat pipe element itself. Accordingly, heat pipes are commonly attached to a solid base plate which is in turn clamped or otherwise brought into intimate contact with the object to be cooled. The base plate in addition to providing structural integrity for the heat pipe arrangement also provides heatsinking capability for drawing the heat away from the heated body and into the heat pipes. Representative examples of heat pipes for electronic and semiconductor cooling applications include those produced by Thermacore Incorporated, 780 Eden Road, Lancaster, PA 17601.
More recently it has been suggested by North and Avedisian in Cornell University technical report E-91-06 entitled "A Heat Pipe for Cooling High Flux Multichip Modules", published Apr., 1991, that a base plate configured as a manifold having multiple flowpaths between heat pipe elements connected to the base plate may offer improved performance over previous base plate designs. The heat pipe of North and Avedisian is illustrated in FIG. 1. With reference to FIG. 1, North and Avedisian report that a base plate 2 containing parallel sets of three wick-lined holes 8 connecting condenser elements 6 was able to dissipate high heat fluxes (greater than 20 W/cm.sup.2) and high total power (greater than 800 W) while maintaining moderate surface temperatures (less than 100.degree. C.), where the heat pipe was operating in an environment having a temperature difference between heat pipe surface and cooling air of approximately 30.degree. C. Significantly, as North and Avedisian report in the above-cited technical paper, the heat pipe and base plate combination are able to achieve high heat fluxes and power dissipation due to large physical size, thereby rendering the heat pipe arrangement generally unsuitable for compact instrumentation or computation electronics.
Although the heat pipe assembly as developed and reported by North and Avedisian improves upon prior heat pipe designs, the reported design continues to suffer from two limitations frequently encountered in the prior art. The first limitation is that high heat fluxes and total power dissipation are generally obtained by having large base plate surface areas through which heat is transmitted via conduction to heat pipe elements mounted on an opposing surface. Alternatively, heat is conducted through a thin wall and into a working fluid residing directly in the base plate and then evaporated and later condensed on the aforesaid condenser elements. The channel design of North and Avedisian improves upon the prior art by essentially configuring the base plate as the evaporator region by providing channels in the base plate so that the working fluid may be locally evaporated and later condensed in the heat pipe condensing region. However, the design is self-limiting by providing no facility for the working fluid to transfer between adjacent condenser bundles formed by the multiple heat pipe elements. Moreover, the design of North and Avedisian does not permit the heat pipe arrangement to be operated in any orientation except with the base plate horizontal. This result stems from the condensate flow being returned via gravity to the evaporator region and hence to the wick-lined holes.
As will be explained in the following detailed description, the present invention significantly improves upon the heat transfer capability of the base plate by increasing the mobility of the evaporated working fluid within the base plate evaporator region. In addition, the present invention permits heat pipe assemblies to be optimized for either horizontal or vertical operation.